1. Field of the Invention
The present invention relates to a power transmission controlling apparatus and more particularly to a power transmission controlling apparatus for use in automotive vehicles and suitable for controlling a power transmission mechanism provided with a main transmission and an auxiliary transmission positioned at the rear stage of the main transmission.
2. Description of the Background Art
The power transmission mechanism of this type is usually provided with a main transmission operable in a plurality of predetermined operation stages, and an auxiliary transmission operable together with the main transmission and changeable in two different stages including high and low speed stages and simple in construction to realize a high peed change ratio.
In the power transmission mechanism thus constructed, the auxiliary transmission is provided in a transfer to be controlled independently from the main transmission and operative in a low speed stage (hereinafter simply referred to as “Low mode”) or a high speed stage (hereinafter simply referred to as “High mode”). When an automotive vehicle such as for example a four-wheel drive vehicle is cruising on bad roads, rocky roads and the like, the auxiliary transmission is operated in the Low mode changed from the High mode to ensure a four-wheel drive operation with a necessary drive power under a high reduction ratio as compared with that of its usual drive operation.
On the other hand, the changing operation between the High and Low modes requires the main transmission to be maintained in its neutral state. For this reason, the power transmission controlling apparatus is provided with an electronic control unit (hereinafter simply referred to as “ECU”) which can reliably control the changing operation between the High and Low modes while acquiring transmission information from the main transmission.
As one of the conventional power transmission controlling apparatuses, well known is an apparatus which is adapted to set allowable operation conditions for judging a cruising speed of an automotive vehicle, a brake condition of the vehicle, shifted range positions selected in the main transmission, and engine revolutions to allow the auxiliary transmission to perform the changing operation of the High and Low modes (hereinafter simply referred to as “H/L change”), and to always check whether or not a time period at which the allowable changing conditions of the H/L change on the above four items continue to be established ends passing more than a predetermined time. The above checking operations render it possible to quickly execute the H/L change only with the continuous judgment on whether or not the conditions of the H/L change are established as being allowable at the request of the mode change initiated by a mode change switch. The known apparatus thus constructed is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-287557.
As additional one of the conventional power transmission controlling apparatuses, there is known an apparatus which is designed to detect a fail state generated out of a neutral state required for the H/L change of the auxiliary transmission due to abnormal states such as a valve stick, and then to prohibit the H/L change of the auxiliary transmission in the transfer at the time of the detection of the fail state. This known apparatus thus constructed is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-336740.
The other prior-art power transmission controlling apparatus is operative to automatically return a transfer changing lever to its original position to be taken before the H/L change when the main transmission is out of its neutral state, for example, shifted in “D-range” even if the transfer changing lever is operated for changing means to perform the H/L change. The apparatus thus constructed is disclosed in Japanese Patent Application Laid-Open Publication No. 10-181373.
In the conventional power transmission controlling apparatuses previously mentioned, the main transmission is apt to be erroneously shifted by a driver to a non-neutral position such as “D-range” after the H/L change operation of the auxiliary transmission is initiated so that there is frequently caused an unallowable operation condition established, i.e., a violated condition for the auxiliary transmission at the time when the auxiliary transmission starts to be operated. As a result, the auxiliary transmission is brought into a state in which the main transmission starts the power transmitting operation before the H/L changing gears forming part of the auxiliary transmission are meshed with each other for the H/L change operation of the auxiliary transmission. Also in the conventional apparatuses one of which is constructed to prohibit the H/L change operation of the auxiliary transmission when the violated condition is established for the auxiliary transmission and the other of which is designed to return the changing members and elements forming part of the auxiliary transmission to their respective original positions to be taken before the H/L change operation, there are caused unpleasant gear noises, thereby lowering a durability of the auxiliary transmission.
To overcome the above problems, there has been proposed information medium for example such as owner's manuals and notice labels to pay attention to the driver on the use of the four-wheel drive vehicle, or a buzzer to be operated when the main transmission is shifted to a non-neutral position during the H/L change operation to prevent the violated condition from being generated or to urge the main transmission to quickly be shifted to the neutral position at the time of the violated condition occurring. These preventive measures and means for overcoming problems as to the gear noises cannot completely eliminate such annoying gear noises from the auxiliary transmission.
To prevent the gear noises from being generated in the auxiliary transmission, it is considered to prohibit the shifting operations of the main transmission during the H/L change operation of the auxiliary transmission, however, such consideration cannot be applied to a manual operation type of power transmission mechanism. Moreover, in the event that the transfer has entered a H/L change state and encountered an abnormal situation to have the auxiliary transmission unable to perform the H/L change operation, it is likely that the transmission operation by the main transmission is difficult to be carried out. The difficult transmission operation thus caused is not suitable for a limp-home function and a fail-safe function to be effected by the main transmission.